The present invention relates to an improved facsimile reception apparatus for receiving and processing transmitted data in compressed form.
Facsimile transmission systems are extremely useful for transmitting original documents in numerous commercial and bureaucratic applications. The original document is scanned to produce electrical signals which are transmitted over a telephone line or the like to a remote receiver. The receiver prints a facsimile or reproduction of the original document in response to the received signals. In order to increase the transmission speed, it is desirable to compress the signals for transmission using run length encoding or the like.
The original document is scanned in an orthogonal pattern of scan lines and the signals transmitted on a line by line basis. Each line of compressed data consists of data bits and fill bits. A sync code is added at the end of each line. In order to minimize the buffer memory requirements of the receiver, each line of data has a predetermined length and the printer at the receiver is constructed to print each line faster than it is compressed and transmitted. Thus, each line of compressed data has a predetermined number of bits. Often, the run length code is quite short, and fill bits, generally consisting of zeroes, are added to the data bits to increase the length of the line to the predetermined value. Printing of a new line is effected in response to detection of a sync code.
A problem has existed heretofore in this type of facsimile system regarding erroneous detection of fill bits as sync codes. This is because where the fill bits consist of zeroes and the sync code consists of several zeroes and ones to the end of the fill bits, transmission noise or errors caused by other phenomenon can alter the fill bits such that they constitute a false sync code.
A false sync code can cause the printer to increment to the next line before finishing printing a current line. Since the data between the false sync code and the true sync code will be printed as a line of data, a line of erroneous data will be printed and the total number of lines on the reproduction will be excessive. This results in considerable distortion. The printer may also malfunction mechanically in response to a false sync code.
One prior art expedient for overcoming this problem is to count the number of bits of data between successive sync codes after expansion of the intervening line. Another expedient is to utilize a timer to measure the length of time between successive sync codes. However, these methods are not practical because of excessive complexity and cost.